sedimentary

Septarian Nodule

Septarian Concretion (Composition: Calcite, Aragonite, and Limestone/Clay)

AI-generated identification · may be incorrect

Hardness: 3.5 to 4 on Mohs scale; Color: Brown, grey, and yellow with dark centers; Luster: Vitreous to earthy; Crystal structure: Trigonal/Orthorhombic (inner minerals); Cleavage: Perfect rhombohedral in calcite portions; Specific gravity: 2.7.

Hardness: 3
Color: Brown, grey, and yellow with dark centers
Luster: Vitreous to earthy

Identified March 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Cretaceous period (approx. 50-70 million years ago) from the accumulation of sedimentary matter around a core. As the mud balls dried and shrank, they cracked, and calcite/aragonite precipitated into the fractures from groundwater.

Uses & applications

Primarily used as ornamental display pieces, home decor, and lapidary crafts (cabochons and bookends). It is also highly valued by mineral collectors for its unique 'dragon skin' appearance.

Geological facts

Also known as 'Dragon Stone' due to its scale-like patterns. These nodules are unique because they represent a combination of biological and chemical processes, often containing fossilized remains at their center.

Field identification & locations

Identify in the field by looking for spherical or potato-shaped mudstone rocks with visible star-shaped cracks. Common locations include Utah (USA), Madagascar, and various parts of the UK. Collectors should look for high contrast between the yellow calcite and dark limestone.